This invention relates to the testing of electrochemical cells during the manufacturing process and, in particular, to improved apparatus for applying and receiving electrical signals during testing of rechargeable type electrochemical cells.
It is accepted practice in the electrochemical cell manufacturing industry to subject each manufactured cell to a test procedure for the purpose of checking cell performance. In the case of electrochemical cells of the rechargeable type, performance is ascertained by measuring the electrical characteristics of the cell after it has been fully charged. The exact nature of these tests varies from one manufacturer to another but, in general, each involves impressing a full electrical charge (or even overcharge) on the cell and, only then, measuring the cell voltage. Thereafter the cell may be discharged to a predetermined level in order to provide a measure of the length of time for discharge or the magnitude of the discharge current.
In one currently followed procedure, rechargeable nickel-cadmium cells are taken from the assembly line and placed in apparatus which charges the cells at a predetermined rate that is a fraction of the one-hour power current rating ("C") for the cell. This charging is carried out for a time sufficient to effect at least a certain degree of overcharge to the cell. For example, a typical sealed nickel-cadmium cell is charged at the 0.1C rate for 24 hours (C being the current rating of the cell at one hour). This results in the cell's being supplied with a charge equal to 240% of its rated capacity, and the average normal cell may be in overcharge for perhaps 5-7 hours in order to ensure that all cells subjected to the ensuing tests are completely charged.
Prior to terminating the 0.1C charging current applied to the cell, the cell voltage is measured. This voltage measurement generally provides an indication of an insufficient electrolyte, plate mismatch, presence of carbonates or a shorted condition of the cell. A cell voltage while the cell is being charged which is abnormally high indicates a low electrolyte, plate mismatch or presence of carbonates condition, whereas a charge voltage which is abnormally low tends to indicate a short.
After this voltage measurement, the cell is subsequently discharged at the rate of 2C (i.e., at a discharge current which is double the one hour current rating C of the cell), and the time needed for the cell to reach a subnominal voltage is recorded. The discharge time provides an indication of several of the characteristics of the cell, including its capacity, normal or abnormally low electrolyte, leaks, and shorted conditions.
Tests now performed on electrochemical cells, by charging and subsequently discharging the cell, generally provide complete information of the cell behavior and are more than adequate to detect most manufacturing faults or other cell defects; but the procedure has many drawbacks. Space and machinery must be provided in the plant for charging, discharging, and conducting measurements on the cells. Since the cells must be charged for up to 24 hours before any kind of measurement is performed, a great deal of storage space must be provided to accept the full 24-hour plant output of cells so that the cells can be loaded into the charger. It is difficult to conduct such charging on cells moving on a production line due to the large number of cells which are produced over the period of 24 hours (this would require an extremely long track to store and to charge moving cells) and, accordingly, charging is usually done on a batch basis whereby a great number of cells are inserted into the charger at the same time for charging.
The second drawback has to do with the discharging of the cell. Adequate electronic circuitry must be provided in order to sense when the cell voltage of each individual cell has attained the predetermined voltage level (e.g., one volt) and then open the discharge path in order that the cell not be put into an overdischarged condition. Discharge of the cell, being at the 2C rate, consumes another thirty minutes of testing time, following which the last series of electrical measurements are made.
From the foregoing, it can be seen that a great deal of storage space, testing machinery and time are consumed in the mere testing of the cells owing to the fact that each manufactured cell must be charged and discharged. The testing procedure becomes, in essence, a bottleneck.
U.S. application Ser. No. 812,727, filed July 5, 1977, of Rameshchandra V. Shah, entitled "Method For Rapidly Testing Quality of Incompletely Charged Electrochemical Cells" and assigned to the assignee of the present invention, describes a testing method for eliminating the production bottleneck of cell testing while providing a reliable check of the most common defects encountered in the mass production of recharging electrochemical cells. All test measurements are performed in one or two seconds at a station on the actual cell assembly line; for example, at a station immediately after the cell has been sealed.
In that method, a relatively large current pulse of short duration is applied to the terminals of the cell as it is moved along the assembly line. Before terminating the applied current pulse, the voltage across the cell terminals is measured and compared against a predetermined level, or levels, representative of satisfactory cell performance.
Preferably one or more other measurements are made at the same station, such as measurements of the open circuit cell voltage prior to, and immediately after, termination of the current pulse.
Essential to practical quick testing of electrochemical cells on the production line is a reliable test means for applying and receiving electrical signals to and from the cell. In order to avoid the necessity of storing cells during an interim between manufacturing and testing, complete testing of the cell should take place at a rate not slower than the rate of manufacture of the cell. The test means should be capable of operating at the same fast rate. Moreover, the test means must be capable of applying and receiving electrical signals in such a manner that positive electrical contact is established reliably from one cell to the next in the manufacturing environment.
Test procedures used heretofore have employed one type of device or another for applying and receiving electrical signals. Sometimes a plurality of cells is lowered into a common apparatus having a plurality of contacts, one set for each cell, wherein the cell remains in a cell holder for a substantial period of time while the test is being carried out. When attempting to test cells at the production rate, difficulty was experienced in establishing reliable electrical contact under varying maufacturing conditions. Some cells, for example, come off the production line with fine coatings of grease, oil, dust or other particles which interfere with the electrical contact between the cell terminals and the terminals of the measuring apparatus. Attempts to rely on leaf-spring type contacts for applying and receiving electrical signals to the cell, wherein the cell pushes against and deflects the contact as it moves into the position in which measurement is to be made were unsatisfactory owing to debris and contamination on the cell casings.
The apparatus of the present invention overcomes the foregoing problems and provides means for making electrical contact with the cell terminals on the production line rapidly and reliably. This electrical contact is established notwithstanding mechanical variations in cell size and/or the presence of surface contamination which heretofore has impeded measurement reliability. Moreover, the apparatus of the invention will tolerate small variations in the position of the cell on the assembly line.